I figured that if I took the watertight SIPs route then that covers the rump of the project that I feel out of my depth with. Regarding the rest, I know enough electricians, tilers, chippies, groundworkers, plumbers etc to muddle through it and hopefully not get too badly burnt. I am pretty good at managing costs and getting things done affordably, that's a big part of keeping the business I run afloat. My problem is I'm spinning enough plates with that, I won't have time to do too much hands on work myself, plus, in all honestly I'm a bit cackhanded. I know my strengths, and accept building isn't one of them.
Thanks for your 2p.